Wavelength conversion element

ABSTRACT

Two grooves  10  are diced in parallel along the light passage direction in a quartz quasi-phase matching element  1 . Consequently, as is shown in (b) and (c), a protruding part  11  which is positioned between the two grooves  10  is formed on the upper surface side (in the figures), and a ridge type waveguide  9  is formed inside this protruding part. Accordingly, if light is caused to pass through this ridge type waveguide  9 , the light can be caused to pass through the portions with inverted crystal axes (polarization inversion regions)  4 , and can be subjected to a wavelength conversion, in a state in which the light is confined into the ridge type wavelength guide  9 . As a result, a state can be produced in which the energy of the light is high inside the wavelength conversion region, so that a high wavelength conversion efficiency can be obtained.

This application is the United States national phase application ofInternational Application PCT/JP2003/011881 filed Sep. 18, 2003.

TECHNICAL FIELD

The present invention relates to a wavelength conversion element that isused to output a wavelength that differs from that of the incident lightby using a quasi-phase matching technique.

BACKGROUND ART

Especially in the field of optical communications, wavelength conversiontechniques that produce light with a wavelength differing from that ofthe incident light by means of higher-order interaction betweensubstances and light have attracted attention. In such wavelengthconversion techniques, methods for efficiently extracting light from theinterior of the material following conversion include:

-   (1) a method which utilizes the birefringence of a crystalline    material, and accomplishes phase matching of the input and output    wavelengths by propagating light at a specified angle; and-   (2) a method called “quasi-phase matching” in which periodic    polarization inversion regions are formed on the light propagation    path, and the difference in the phases of the input and output    wavelengths is eliminated in approximate terms.

Of these two methods, the latter quasi-phase matching would appear topossess numerous advantages in adaptation for practical use, e.g., thepermissible width of the operating wavelength and angle of incidence islarge, the phenomenon known as “walk-off” in which the input and outputlights travel along different directions does not occur, and the like;accordingly, this method has been the focus of various expectations.

The formation of a polarization inversion region in a wavelengthconversion element utilizing a quasi-phase matching technique can berealized (for example) by using a ferroelectric material such as lithiumniobate as the substrate material, patterning an electrode in the regionwhere it is desired to accomplish polarization inversion using aphotolithographic technique, and applying a high voltage to thiselectrode, so that partial inversion of the crystal axes is accomplishedby means of the electric field.

Besides such a method in which a polarization inversion region is formedby applying a voltage to a ferroelectric material, a wavelengthconversion element in which a polarization inversion region is formed byusing quartz (which is not a ferroelectric material) as the substrate,and applying stress, has been proposed in recent years (S. Kurimura, R.Batchko, J. Mansell, R. Route, M. Fejer and R. Byer: 1998 Spring Meetingof the Japan Society of Applied Physics, Proceedings 28a-SG-18).

This wavelength conversion element using quartz as the substratematerial shows a light resistance that is at least 100 times greaterthan that of an element using a ferroelectric material as the substrate.Furthermore, the lower-limit wavelength at which the element istransparent is around 150 nm, while the same wavelength is 350 nm in thecase of lithium niobate. Consequently, the following advantage isobtained: namely, light at wavelengths that conventionally could not beused, and in particular, even light at a wavelength of approximately 193nm, which is comparable to that of an ArF excimer laser, can also beused.

Incidentally, wavelength conversion techniques are based on the mutualinteraction of higher-order light and substances, and in order to obtaina high conversion efficiency, it is desirable that the energy density ofthe light within the wavelength conversion element be high. In caseswhere lithium niobate, which is a ferroelectric material, is used as thewavelength conversion material, a method that is widely practiced as amethod that uses light with a high energy density is a method called the“proton exchange method” in which the refractive index is raised byreplacing some of the lithium in the substrate with protons inhigh-temperature molten benzoic acid as shown below. This is a method inwhich a portion with a high refractive index is formed in the substrateby the proton exchange method, a waveguide is formed in this portion,and light is confined into this waveguide.LiNbO₃+(C₆H₅COOH)_(x)→Li_(1−x)H_(x)NbO₃+(C₆H₅COOLi)_(x)

In concrete terms, after a polarization inversion region is formed bythe application of an electric field, an aluminum thin film is formed onthe surface of the substrate, with the region in which it is desired toform a waveguide left “as is”. The formation of the aluminum thin filmis accomplished by an ordinary lift-off process. After being masked withaluminum, the substrate is immersed in benzoic acid heated to atemperature of 350° C. to 400° C., and is allowed to stand for aspecified time, so that the proton exchange process is promoted.Following this proton exchange, the aluminum is removed by etching. Theregion into which protons have been exchanged has a higher refractiveindex than the surrounding regions, so that an optical waveguide inwhich light is confined and propagated is formed. Thus, light can beconfined and propagated inside a periodic polarization inversion region,so that a high conversion efficiency can be obtained.

However, in the case of a quasi-phase matching element using quartz, thefollowing problem is encountered: namely, waveguides cannot be formedusing such a proton exchange process, so that the confinement of thelight is impossible; as a result, a high conversion efficiency cannot beobtained.

The present invention was devised in light of such circumstances; theobject of the present invention is to provide a wavelength conversionelement in which a waveguide that confines light can be formed, so thata high wavelength conversion efficiency can be obtained, even in aquasi-phase matching element using quartz.

DISCLOSURE OF THE INVENTION

The first invention that is used to achieve the object described aboveis a wavelength conversion element in which a plurality of polarizationinversion regions are formed in a quartz crystal substrate in a periodicmanner, and light that is incident from one end of the quartz crystalsubstrate is subjected to a wavelength conversion by being caused topass through the plurality of polarization inversion regions, thiswavelength conversion element being characterized in that ahigh-refractive-index region is formed so that this region passesthrough the plurality of polarization inversion regions in the directionof light transmission.

In this invention, since a high-refractive-index region is formed so asto pass through the plurality of polarization inversion regions in thedirection of light transmission, light can be propagated through thepolarization inversion regions in a state in which the light is confinedinto the high-refractive-index region by introducing the light into thishigh-refractive-index region. Accordingly, a wavelength conversionelement with a high light conversion efficiency can be obtained.

The second invention that is used to achieve the object described aboveis the first invention, which is characterized in that thehigh-refractive-index region is formed by converting the area aroundthis region into a low-refractive-index region by means of ionimplantation.

By performing ion implantation so that the area surrounding the regionthat is desired to be converted into a high-refractive-index region isset at a low refractive index, it is possible to raise the refractiveindex of this portion in relative terms. A fine high-refractive-indexregion can be formed by combining the present invention and aphotolithographic technique, and light can be propagated while beingcinfined into this region.

The third invention that is used to achieve the object described aboveis the first invention, which is characterized in that thehigh-refractive-index region is formed by a ridge type waveguide.

The term “ridge type waveguide” refers to a device in which a protrudinghigh-refractive-index part is disposed on a low-refractive-index part,and this protruding high-refractive-index part is used as an opticalwaveguide. In the present invention as well, light can be propagated ina state in which the light is cinfined into a protrudinghigh-refractive-index part. Furthermore, this invention differs from thesecond invention in that the system can be devised so that ionimplantation is not performed in the polarization inversion regions; insuch a case, therefore, there is no danger that the characteristics ofthe polarization inversion regions will be altered.

The fourth invention that is used to achieve the object described aboveis the third invention, which is characterized in that the ridge typewaveguide is formed by selective reactive ion etching.

The ridge type waveguide can be formed by etching the quartz crystalsubstrate by selective reactive ion etching. If this invention is usedin combination with a lithographic process, a fine ridge type waveguidecan be formed.

The fifth invention that is used to achieve the object described aboveis the third invention, which is characterized in that the ridge typewaveguide is formed by mechanical working.

A ridge type waveguide can also be formed in the quartz crystalsubstrate by mechanical working such as dicing. In this invention, aridge type waveguide can be formed by a relatively simple process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)–1(c) are diagrams illustrating the schematic construction ofa quasi-phase matching element using quartz, which is the material of awavelength conversion element constituting a working configuration ofthe present invention.

FIGS. 2( a)–2(e) a diagrams which show the process used to manufacture awavelength conversion element constituting a working configuration ofthe present invention by ion implantation using the quartz quasi-phasematching element shown in FIG. 1( a) as a substrate. The drawings on theleft side are sectional views along line A—A in FIG. 1( a), and thedrawings on the right side are sectional views along line B—B in FIG. 1(a).

FIGS. 3( a)–3(e) are diagrams which show the process used to manufacturea wavelength conversion element constituting a working configuration ofthe present invention by constructing a ridge type waveguide byselective reactive ion etching in the quartz quasi-phase matchingelement shown in FIG. 1( a) as a substrate. The drawings on the leftside are sectional views along line A—A in FIG. 1( a), and the drawingson the right side are sectional views along line B—B in FIG. 1( a).

FIGS. 4( a)–4(c) are diagrams showing a wavelength conversion elementconstituting a working configuration of the present invention, in whicha ridge type waveguide is constructed by dicing using the quartzquasi-phase matching element shown in FIG. 1( a) as a substrate.

BEST MODE FOR CARRYING OUT THE INVENTION

A working configuration of the present invention will be described belowwith reference to the figures. FIG. 1 is a diagram showing the schematicconstruction of a quasi-phase matching element using quartz, which isthe material of a wavelength conversion element constituting a workingconfiguration of the present invention. FIG. 1( a) is a plan view, FIG.1( b) is a sectional view along line A—A in FIG. 1( a), and FIG. 1( c)is a sectional view along line B—B in FIG. 1( a).

Protruding parts 2 are disposed in a periodic manner on one surface of aquasi-phase matching element 1. The quasi-phase matching element 1 whichuses quartz is formed by a hot pressing method. Specifically, the quartzcrystal substrate of this element which has periodic protruding parts 2on one surface as shown in FIG. 1 is clamped by heater blocks from aboveand below; the temperature of the quartz crystal substrate is elevated,and at the point in time at which the desired temperature is reached,pressure is applied. In this case, since a stress acts only on theportions corresponding to the protruding parts 2, the crystal axiscomponents are inverted only in these portions. These portions withinverted crystal axes grow into the interior of the crystal, and arethus propagated into the interior of the crystal, so that these portionsare incorporated to a considerable extent in the direction of depth ofthe substrate main body part 3. The portions with inverted crystal axes(polarization inversion regions) are indicated by the symbol 4 in thediagram. Thus, a periodic twin lattice can be manufactured inside thesubstrate main body part 3.

Below, the process whereby a wavelength conversion element constitutinga working configuration of the present invention is manufactured by ionimplantation using such a quartz quasi-phase matching element 1 as asubstrate will be described with reference to FIG. 2. FIG. 2 showssectional views along line A—A and sectional views along line B—B inFIG. 1( a). Furthermore, in the following figures, constituent elementsthat are the same as the constituent elements in the figures describedabove are labeled with the same symbols, and a description of suchelement may be omitted.

In FIG. 2, FIG. 2( a) is a diagram which shows the quartz quasi-phasematching element 1. A positive type resist layer 5 having a thicknesssufficient to cover the protruding parts 2 is formed on the surface ofthis quasi-phase matching element 1 (FIG. 2( b)). Then, the resist isexposed by lithography so that the central portion in the cross-sectionalong line B—B of the quasi-phase matching element 1 is left “as is”,and the resist is developed, so that a portion in which the resist 5remains is formed in the central part (FIG. 2( c)).

Next, using the resist 5 as a mask, He ions are implanted in the surfaceof the quasi-phase matching element 1. As a result, low-refractive-indexregions 6 a are formed in the portions where the resist 5 is absent(FIG. 2( d)).

Subsequently, the resist layer 5 is removed, and He ions with adifferent energy from those used in the step shown in FIG. 2( d) areimplanted. Specifically, the energy of the He ions implanted in thiscase is increased, so that the ions do not stop in portions that areclose to the surface of the substrate, but instead accumulate at acertain depth. Consequently, a low-refractive-index region 6 b is newlyformed in a position located at a specified depth (FIG. 2( e)).

In this state, since a low-refractive index region is not formed in theportion with a shallow depth located immediately beneath the resist 5 inFIG. 2( d), this portion forms a high-refractive-index region 7 whoserefractive index is relatively higher than that of the surroundingareas. As a result, the wavelength conversion element constituting aworking configuration of the present invention is completed.

In this wavelength conversion element, the high-refractive-index region7 is formed so that this region passes through the plurality of portionswith inverted crystal axes (polarization inversion regions) 4.Accordingly, if light is caused to pass through thishigh-refractive-index region 7, the light can be caused to pass throughthe portions with inverted crystal axes (polarization inversion regions)4, and can be subjected to a wavelength conversion, in a state in whichthe light is confined into the high-refractive-index region 7.Consequently, a state can be produced in which the energy of the lightis high inside the wavelength conversion element, so that a highwavelength conversion efficiency can be obtained.

A process used to manufacture the wavelength conversion elementconstituting a working configuration of the present invention byconstructing a ridge type waveguide by selective reactive ion etchingusing the quartz quasi-phase matching element 1 shown in FIG. 1 as asubstrate will be described below with reference to FIG. 3. FIG. 3 showssectional views along line A—A and sectional views along line B—B inFIG. 1( a).

In FIG. 3, FIG. 3( a) is a diagram showing the quartz quasi-phasematching element 1. A negative type resist layer 5 with a thicknesssufficient to cover the protruding parts 2 is formed on the surface ofthis quasi-phase matching element 1 (FIG. 3( b)). Then, the centralportion of the quasi-phase matching element 1 in the sectional viewalong line B—B is exposed by lithography over a specified width in theleft-right direction in FIG. 1, and the resist is developed, so that theresist is removed with the exposed portion left “as is” (FIG. 3( c)).

Next, using the resist as a mask, a CF4+H2 type gas is used in thesurface of the quasi-phase matching element 1. As a result, when thesurface of the substrate is etched, the portions covered by the resistremain so that a protruding part 8 is formed (FIG. 3( d)). Subsequently,the wavelength conversion element constituting one working configurationof the present invention is completed by removing the resist layer 5(FIG. 3( e)).

The protruding part 8 is formed so as to pass through the plurality ofportions with inverted crystal axes (polarization inversion regions) 4,and the ridge type waveguide 9 is formed in the lower part of thisprotruding part. Accordingly, light can be caused to pass through theportions with inverted crystal axes (polarization inversion regions) 4,and thus subjected to a wavelength conversion, in a state in which thelight is confined inside the ridge type waveguide 9, by causing thislight to pass through the ridge type waveguide 9. Consequently, a statecan be produced in which the energy of the light is high inside thewavelength conversion element, so that a high wavelength conversionefficiency can be obtained.

Below, a wavelength conversion element constituting one workingconfiguration of the present invention in which a ridge type waveguideis constructed by dicing in the quartz quasi-phase matching element 1shown in FIG. 1 as a substrate will be described with reference to FIG.4. In FIG. 4, FIG. 4( a) is a plan view, FIG. 4( b) is a sectional viewalong line A—A in FIG. 4( a), and FIG. 4( c) is a sectional view alongline B—B in FIG. 4( a).

The wavelength conversion element shown in FIG. 4 is formed by forminggrooves by means of dicing in the quartz quasi-phase matching element 1shown in FIG. 1. Specifically, two grooves 10 are diced in parallelalong the light passage direction. Consequently, as is shown in FIGS. 4(b) and 4(c), a protruding part 11 which is positioned between the twogrooves 10 is formed on the upper surface in the figures, and a ridgetype waveguide 9 is formed inside this protruding part. Accordingly,light can be caused to pass through the portions with inverted crystalaxes (polarization inversion regions) 4, and thus subjected to awavelength conversion, in a state in which the light is confined insidethe ridge type waveguide 9, by causing the light to pass through thisridge type waveguide 9. Consequently, a state can be produced in whichthe intensity of the light is high inside the wavelength conversionelement, so that a high wavelength conversion efficiency can beobtained.

In this method, since the formation of the ridge type waveguide 9 isaccomplished by mechanical working, the process is simple; furthermore,since there is no use of ion implantation, etc., this method ischaracterized by the fact that there is no alteration of the propertiesof the portions with inverted crystal axes (polarization inversionregions) 4.

Furthermore, in FIGS. 3 and 4, the low-refractive-index substance thatis used to form the ridge type waveguide 9 is air; however, it wouldalso be possible, for example, to cover the upper surface of thewaveguide conversion element shown in FIGS. 3 and 4 with a substancethat has a lower refractive index than quartz.

1. A wavelength conversion element in which a plurality of polarization inversion regions are formed in a quartz crystal substrate in a periodic manner, and light that is incident from one end of the quartz crystal substrate is subjected to a wavelength conversion by being caused to pass through the plurality of polarization inversion regions, this wavelength conversion element being characterized in that a high-refractive-index region is formed so that this region passes through the plurality of polarization inversion regions in the direction of light transmission, and wherein the high-refractive-index region is formed by converting the area around this region into a low-refractive-index region by means of ion implantation. 